This invention relates to a plotter or a similar device wherein sheet material from a supply roll is moved over a supporting surface and figures, characters and other graphics are drawn segment-by-segment on the sheet material by a drawing instrument as the sheet material is intermittently advanced segment-by-segment over the supporting surface, and deals more particularly with a simple arrangement, including a brake for the supply roll, assuring accurate positioning of the sheet material at the end of an advancement, and the maintenance of that position until the start of the next advancement, to secure an accurate matching of the drawing segment drawn on each sheet material segment with the drawing segments drawn on adjacent sheet material segments.
In the plotter of copending patent application Ser. No. 195,128 paper is pulled from the supply roll by a gear motor which drives a paper take-up roll. A displacement sensing disk engages the paper, is rotated as the paper moves and has its rotational displacement sensed to provide an indication of the paper displacement. During an advancement, when the disk indicates the advancement is nearly completed the speed of the take-up motor is slowed, and then when the disk indicates the advancement is entirely complete the take-up motor is stopped. For good matching of the drawing segment-by-segment, at the instant the take-up motor is stopped the movement of the paper should also stop and its position relative to the supporting surface at that instant should also be held until the start of the next advancement.
In the copending patent application Ser. No. 195,128 a friction disk engaging one end of the supply roll is disclosed for applying a force opposing the unwinding of paper from the supply roll so that the paper is kept in tension as it is pulled off of the supply roll and so that the supply roll will stop immediately without overshoot when the take-up motor stops. When the take-up motor does stop, a certain amount of energy is stored in spring-like fashion between the rotor of the take-up motor and the segment of paper on the supporting surface. Under good conditions this stored energy is resisted by the friction force applied to the supply roll so that the stretch of paper existing between the rolls remains stationary and in tension. However, if the friction force applied to the supply roll is initially too small, or if such friction force though initially sufficient becomes lost, as through vibration of the plotter, the stored energy may act to draw a small amount of additional paper from the supply roll and thereby shift the paper a small amount relative to the supporting surface causing it to lose its previously accurately determined position.
In particular, it has been found that the effectiveness of the friction disk of the copending application relies to a great degree upon the ability to have the side edges of the wound paper of the supply roll aligned with one another to form a uniformally flat end surface for the roll. But, commercially wound rolls of paper often do not have perfectly flat end surfaces. Therefore, the desired friction force may not always be developed, and further if the end surface of the roll is momentarily displaced from the friction disk, through vibration of the plotter, the friction force may be partially or entirely lost.
Accordingly, it is an object of the present invention to provide in a plotter such as aforesaid means by which paper or similar sheet material may be pulled from the supply roll under tension independently of the existence of a perfectly flat end surface on the roll, and whereby at the end of an advancement the sheet material movement is stopped simultaneously with the stopping of the take-up motor and its position held fixed until the start of the next advancement against the force of energy stored between the paper on the support surface and the take-up motor which stored energy holds the stretch of paper existing between the two rolls in tension.
Another object of the invention is to provide a simple friction brake construction for use with the supply roll which in cooperation with the other parts of the plotter during an advancement exerts a retarding or reverse biasing force on the supply roll causing the paper to be pulled from the supply roll under tension, and which at the end of an advancement continues to apply a reverse bias to the supply roll sufficient to hold the paper stationary and in tension against the spring forces stored in the drive train between the paper on the support surface and the rotor of the take-up motor.
Other objects and advantages of the present invention will be apparent from the following description of a preferred embodiment and from the accompanying drawings and claims.